Cargill Township carbonatite complex, District of ... - Geology Ontario

More documents

Recommendations

Info

CARBONATITE - ALKALIC ROCK COMPLEXES: CARGILL TOWNSHIP MAFIC TO ULTRAMAFIC RIM ROCKS Allen (1972, p.38) (Appendix B) subdivided the ultramafic to mafic rocks of the rim into olivine clinopyroxenite, olivine-rim magnetite clinopyroxenite, magnetite-oligoclase clinopyroxenite, hornblendite, hornblende-oligoclase pyroxenite, soda-pyroxene hornblendite, late stage veins and segregations, phlogopite clinopyroxenite, carbonatite, and ultramafic hybrid rocks. The author observed most of these rock types in the suite examined. Hornblende-oligoclase clinopyroxenite and soda-pyroxene hornblende were not observed by the author. Late stage veins and segregations were noted as a minor constituent of the dia mond drill cores but were not examined in thin section. Phlogopite clinopyroxenite was noted in several places on the complex as weathered slump at surface. Its coarse grained and deeply weathered nature made sampling for either thin sectioning or chemical work impractical. The author found the following rock types in rocks of the pyroxenide border of the complex: amphibole-olivine clinopyroxenite; olivine-phlogopite-bearing clinopyroxenite, amphibole rock; clinopyroxenite; amphibole clinopyroxenite; and biotite-carbonate amphibole rock. This rock suite is believed to be roughly equivalent to the first four lithologies described by Allen (1972, p.38) as listed above. Clinopyroxene-Rich Rocks The pyroxene is a clinopyroxene whose extinction angle is dominantly character istic of diopside though some extinction angles characteristic of augite were noted. The grains are euhedral to anhedral in form with anhedral to subhedral forms dominating. The grains are equant, colourless, and generally lack notice able pleochroism. One thin section displays a faint variation in colour from core to rim implying some variation in composition. The pyroxene commonly displays a schiller structure due to oriented inclusions. Allen (1972, p.44) determined that the inclusions are titanomagnetite and that they are oriented along the (100), (010) and (001) crystallographic planes of the pyroxene. Twinning of the crystals is common and Allen (1972, p.44) determined that the twinning is on the (100) crystallographic plane. Allen (1972, p.44) reported that generally the centres of the large crystals are exsolution free, and inclusions become more abundant to wards the margins of the crystal which define relict euhedral growth zones de stroyed by later intercumulus overgrowth. One thin section examined by the author displays amphibole and pyroxene complexly intergrown with sharp boundaries. The author interprets this texture to result from simultaneous crystal lization of the two phases. Allen (1972, p.43) reported the presence of a hiatal texture in the pyroxene and evidence for formation of pyroxene plus magnetite by reaction of olivine and liquid. Zoning of the pyroxene with an increasing iron content from interior to rim becomes more common as the proportion of intersti tial magnetite increases (Allen 1972, p.43). Allen (1972, p. 127) has defined two trends in the pyroxene composition. One trend is without soda enrichment and the other is with soda enrichment. The earliest or more primitive pyroxene is rich in calcium and the latest is rich in sodium. The direction of sodium enrichment is from olivine clinopyroxenite to the soda-pyroxene hornblendite. The reader should refer to Allen (1972, p. 125-137) for a detailed discussion of pyroxene chemistry. Olivine occurs interstitial to the clinopyroxene. The olivine occurs in inter locking aggregates of polygonal, equant, densely-packed grains that are irregular, elongate, or lobate in form. The crystal faces of the clinopyroxene are somewhat 10
R. P. SAGE more euhedral when in contact with the olivine than with each other. The olivine varies from fresh to completely altered. The alteration ocurs along grain bounda ries and along fractures within the olivine. Serpentine, iddingsite, and bowlingite appear to be the alteration products. Allen (1972, p.41) also reported the pres ence of talc. A very minor amount of olivine occurs as a narrow zone or band separating magnetite and clinopyroxene. These bands are discontinuous and likely represent intercumulus growth. Allen (1972) classified rocks displaying this feature as olivine-rim magnetite clinopyroxenite. On the basis of microprobe data Allen (1972, p.49) determined that the forsterite content of the olivines interstitial to the clinopyroxene varied from 65.4 to 76.096. The narrow zones or bands of olivine between the magnetite and clinopyroxene vary from 62.5 to 70.89fc forsterite (Allen 1972, p.49). Amphibole occurs as anhedral to rarely subhedral brown or greenish-brown grains. It is common as irregular patches scattered throughout the clinopyroxene. The patchy replacements are crystallographically compatible with, and merge in a diffuse manner into the pyroxene and reflect late stage alteration of the pyroxene. Crystallographic controlled structures within the pyroxene have obvi ously controlled the replacement by the amphibole. Amphibole is also common as rims of varying widths mantling the pyroxene grains. It is also common as a narrow zone along magnetite-pyroxene contacts and greatly exceeds olivine which also occurs as narrow zones along magnetite-pyroxene contacts. One thin section contains a clot of carbonate into which euhedral amphibole crystals project. The amphibole crystals lie along the edge of the pyroxene crystals which enclosed the carbonate clot. The crystals have brown to greenish brown bases and pale green rims. The amphibole crystals are strongly zoned and occur with a small subhedral grain of sphene. Microprobe analyses by Allen (1972, p.52, p. 138-146) indicate that the am phibole is paragasite or ferroan paragasite and that the most sodic varieties are hastingsite. Allen (1972, p.46) considered the amphibole to be primary and the result of crystallization of late-stage interstial residual liquids. The author partly concurs with this interpretation on the basis of textures observed in thin section. Primary amphibole is likely represented by those grains that lie in sharp con tact with other mineral phases and do not display grain morphology or structures that can be traced from pyroxene to the amphibole. Some amphiboles, which display structures traceable from pyroxene into the amphibole across grain boundaries that may be diffuse in appearance, are clearly secondary. This secon dary amphibole may result from reaction of late-stage deuteric fluids with previ ously crystallized pyroxene or may result from reaction of decanted fluids (fenit ization) associated with the emplacement of the later carbonatite magma. All samples contain 59fc or more magnetite, although the author did not use magnetite as a modifier in naming the rocks. Magnetite occurs as anhedral grains throughout the rocks. It is present as an alteration of olivine and commonly displays an affinity for amphibole which may be either primary or secondary in origin. Some of the grains are elongate, lobate, and interstitial to the pyroxene. The textural relations suggest that a major portion of the magnetite is intercumulus and primary in origin. Amphibole, and rarely, olivine occur as thin rims between the pyroxene and magnetite and some of this magnetite is likely secondary resulting from the breakdown of pyroxene or oli- 11
Page 1 and 2: THESE TERMS GOVERN YOUR USE OF THIS
Page 3 and 4: Ontario Geology of Ministry of Mine
Page 5: Foreword The Cargill Township Carbo
Page 8 and 9: TABLES 1. Table Of Lithologic Units
Page 11 and 12: Geology of Carbonatite - Alkalic Ro
Page 13 and 14: Introduction The Cargill Township C
Page 15 and 16: R. P. SAGE Township and extends int
Page 17 and 18: General Geology The Cargill Townshi
Page 19: R. P. SAGE A crandallite-rich blank
Page 23 and 24: Other Mafic to Ultramafic Rocks R.
Page 25 and 26: R. P. SAGE p. 116) as a mixture of
Page 27 and 28: R. P. SAGE cores to magnesio-arfved
Page 29 and 30: R. P. SAGE mined thickness. Sandvik
Page 31 and 32: TABLE 2. STRATIGRAPHY OF OVERBURDEN
Page 33 and 34: R. P. SAGE containing approximately
Page 35 and 36: INTERNAL STRUCTURES R. P. SAGE The
Page 37 and 38: Economic Geology The Cargill Townsh
Page 39 and 40: R. P. SAGE Thoburn (1984) prepared
Page 41 and 42: Appendix A Petrographic Description
Page 43 and 44: R. P. SAGE Clinohumite is anhedral
Page 45 and 46: R. P. SAGE with brown cores and red
Page 47 and 48: R. P. SAGE hedral grains disseminat
Page 49 and 50: R. P. SAGE pyroxene and occurs alon
Page 51 and 52: R. P. SAGE grains disseminated thro
Page 53 and 54: TABLE A-2. MAJOR ELEMENT ANALYSES (
Page 55 and 56: TABLE A-2. CONTINUED. Ref. No. SiO2
Page 57 and 58: TABLE A-2. CONTINUED. Ref. No. SiO2
Page 59 and 60: R. P. SAGE TABLE A-3. TRACE ELEMENT
Page 61 and 62: TABLE A-3. CONTINUED. Ref. No. Ag A
Page 71 and 72:
TABLE A-4. CONTINUED. Ref. No. QZ C
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
R.P. SAGE TABLE A-5. AVERAGE CHEMIC
Page 83 and 84:
Appendix B — Electron Microprobe
Page 85 and 86:
R.P. SAGE TABLE B-2. MICROPROBE MIN
Page 87 and 88:
R. P. SAGE TABLE B-3. MICROPROBE MI
Page 89 and 90:
TABLE B-5. MICROPROBE MINERAL ANALY
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
References Allen, C. 1972: The Petr
Page 99 and 100:
Index Aeromagnetic data, 7, 21, 24
Page 101 and 102:
Kinking Biotite, Rauhaugite, 18 Phl
Page 103:
Chart A — Cargill Carbonatite Com
show all

Cargill Township carbonatite complex, District of ... - Geology Ontario

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?